This invention relates to a process for the production of a fuel gas and more particularly to the single-step catalytic conversion of a methanol feedstock to synthetic natural gas.
Natural gas produced from oil wells located remotely from the ultimate user of the energy source was for many years burned or used at the site. With the developing shortage of fuel, attempts have been made to liquify the natural gas and to ship it in refrigerated tankers to the ultimate user. Such "flared gas" is however expensive to liquify and ship. It has been suggested that this gas might be more economically used by first converting it to methanol. The technology for doing this presently exists by reforming natural gas, primarily methane, with steam to form a synthetic gas, which may be carried out over a nickel catalyst. The resultant synthesis gas can be converted over catalysts such as zinc oxide-chromium oxide to produce methanol. The crude methanol can then be transported in conventional tankers to remote sources where it can be directly used for many purposes.
In addition, the growing shortage of oil and natural gas has placed greater importance on coal as a fuel source. However the direct use of coal presents many problems to its use as a fuel source. Therefore it is desirable to convert coal to other forms of fuel, which are more compatible with present day fuel consumption devices, which can be more easily handled, and which lead to fewer adverse ecological consequences. Coal can be subjected to partial oxidation in the presence of steam to produce a synthetic gas which can then be converted to methanol.
One proposed method for utilization of the methanol obtained from natural gas or coal is to reform it to produce synthetic natural gas. A conventional method for reforming the methanol is to do so at pressures of from about 100 to 300 psig to produce a product gas containing steam, methane, hydrogen, carbon monoxide and carbon dioxide. This product is then subject to reaction over a methanation catalyst to produce a synthetic natural gas consisting primarily of methane with a minor amount of hydrogen. This prior art two-step conversion can be represented as follows: EQU CH.sub.3 OH.fwdarw.(a)CH.sub.4 +(b)H.sub.2 +(c)CO+(d)CO.sub.2 +(e)H.sub.2 O EQU 3H.sub.2 +CO.fwdarw.CH4+H.sub.2 O
Another proposed method for making synthetic natural gas is to steam reform naphtha. The naphtha and steam are passed over a nickel containing catalyst bed to produce a mixture of methane (about 25% by volume), carbon monoxide, carbon dioxide and hydrogen. Thereafter, one or two steps of catalytic methanation are required to produce a synthetic natural gas having a higher percentage of methane.
Indeed, an extensive amount of literature exists on the production of methane from naphtha and other petroleum hydrocarbons. In general, the naphtha is converted at a high temperature over a nickel catalyst to a methane-rich gas which is then (after several optional intermediate steps) passed over a methanation catalyst at a lower temperature to bring about the formation of further amounts of methane by reaction between carbon dioxide, carbon monoxide and hydrogen present in the gas. The process may comprise passing a mixture of preheated hydrocarbons in vapor form and steam at atmospheric or superatmospheric pressure through a bed of a nickel catalyst such that the bed is maintained at temperatures in about the range 450.degree. C. to 750.degree. C. The resultant gases contain steam, hydrogen, carbon dioxide, carbon monoxide and methane. An example of such a process is to pass naphtha with steam (i.e., H.sub.2 O/carbon mole ratio of 1.5 to 2.0) over a nickel catalyst at a pressure of 150 psig and an outlet gas temperature of 640.degree. C. to provide a product having the following analysis (dry basis): methane 34.2%, hydrogen 38.7%, carbon monoxide 11.8% and carbon dioxide 13.3%. The amount of methane in this process is increased by passing the gaseous mixture over a second and optionally a third catalyst bed. The gases are usually cooled prior to methanation to a temperature which is sufficiently low for methane synthesis to occur to a substantial extent but which is not so low that there is insufficient catalytic activity for reaction to proceed at an adequate rate. In general, the temperature of the methanation reaction is within the range of about 200.degree.-400.degree. C. and at a pressure above that of the first stage, i.e. at 20-50 atmospheres or higher. This procedure can be used to change the first stage gaseous mixture to one having a final analysis (on a dry basis, and after scrubbing) of methane 96%, hydrogen 3.6% and carbon dioxide 0.4%. The methanation stages may be used, particularly to obtain a product having a greater percentage of methane. In such a process the gas from the first stage methanation is cooled prior to the second stage methanation since as methane is produced the temperature of the gases rise, i.e., the reaction is exothermic.
While it might be possible to utilize some of this technology in synthesizing methane from methanol, it shares with the above-described synthetic natural gas processes the marked disadvantage of requiring a minimum of two reaction stages to achieve the high methane/low hydrogen content of natural gas. This, of course, places demands on the equipment requirements.
It is accordingly an object of this invention to provide a one-step process for converting a methanol feed to a fuel gas containing a high proportion of methane.
A further object is to provide a process whereby synthetic natural gas can be produced from methanol in a one-step catalytic reaction.
Yet another object of this invention is to provide a process for converting a feed of methanol and naphtha to synthetic natural gas.
These and other objects will become apparent from the detailed description which follows.